Damascene processing is a method for forming interconnections on integrated circuits that involves formation of inlaid metal lines in trenches and vias formed in a dielectric layer (inter-metal dielectric). The metal conductive lines are formed by an electroplating process. Because copper or other mobile conductive material provides the conductive paths of the integrated circuit, the underlying silicon devices must be protected from metal ions (e.g., Cu2+) that might otherwise diffuse or drift into the silicon. Suitable materials for diffusion barrier include tantalum, tantalum nitride, tungsten, titanium tungsten, titanium nitride, tungsten nitride, and the like.
After diffusion barrier is deposited and prior to electroplating, a seed layer of copper or other metal is typically applied by a physical vapor deposition (PVD) process to enable subsequent electrofilling of the features with copper inlay. In order to serve as a seed for electroplating, the seed layer should be continuous, stable and have good adhesion to the barrier layer.
One difficulty with depositing seed layers is that the copper (or other metal) may agglomerate on the barrier layer surface. This phenomenon occurs because of the weak chemical bond between seed and the barrier as a result of the absence of an intermetallic reaction between the two. High oxidation ability of the barrier aggravates this phenomenon. Because of this agglomeration, the copper may not cover the surface in a conformal manner. The thickness of the seed layer coverage is thus uneven, thicker in some places than others, and the layer may include gaps. Such seed layers do not provide a uniform layer for electroplating, which leads to voiding defects in the electroplated copper. Another difficulty is the formation of “overhang” material on the edges of the features. Diffusion barrier overhang, for example, can prevent seed metal deposition flux from reaching the top sidewalls of the features. This phenomenon also leads to seed layers having poor continuity. Defects, poor continuity and poor adhesion of the seed layer all may adversely affect performance of the fabricated device. For example, defects in the seed layer may result in electromigration and stress migration at the seed-barrier interface. Void formation in the electrofilled copper features and subsequent post CMP defects are other consequences of poor seed layer formation that may also lead to device failure.
What are therefore needed are methods of depositing metal seed layers on diffusion barriers that provide seed layers having improved continuity and adhesion to the underlying diffusion barrier.